Resolution and sensitivity of NMR spectroscopy is substantially improved at higher resonance frequencies and progression in high resolution instruments has continued in this direction. At frequencies in excess of a few hundred MHz, discrete reactances in the RF subsystem become inappropriate elements for the resonant circuit and cavity resonators are preferred for the requirements of NMR spectroscopy.
In the prior art, Schneider and Dullenkopf proposed a slotted tube resonator (Rev. Sci. Inst., Vol. 48, No. 1, p. 68-73, January 1977) to achieve a sample irradiation cavity capable of supporting mechanical sample spinning and exhibiting high Q together with reasonable RF homogeneity. This structure comprised two symmetrical arcuate portions of a cylinder, axially elongate and coaxially mounted within an outer tube. The sample is disposed on axis surrounded by the arcuate portions which form a strip line. The slot(s) between the arcuate portions define capicitances in series with a net single turn inductance.
Another NMR resonator of prior art is described by Hardy and Whitehead (R.S.I., Vol. 52, No. 2, p. 213-216, February 1981) and is structurally a split ring forming a single turn inductance in parallel with a capacitance provided by the gap in the ring and coaxially disposed within an outer cylinder capable of supporting a TE propagation mode.
Yet another example of prior art is described by Hyde et al, U.S. Pat. No. 4,435,680. In one embodiment this resonator comprises an inductive element formed from arcuate portions arranged to define a plurality of gaps on a circumferential locus. The inductor is disposed coaxially within an outer shielding cylinder.
As in all NMR instruments, there are certain considerations which necessarily influence the apparatus design. It is important for the signal detection circuits or probe to exhibit adequate sensitivity and signal-to-noise ratio. For studies of magnetic moments of dilute concentration, e.g., C.sup.13, adequate sensitivity will be quite high and incremental improvement in sensitivity is often sought. Contributing to this criteria is a desirability of a high filling factor for efficient volume irradiation capability in order to produce sufficient signal strength.
Additionally, the Q value of the probe circuits is desirably large because the signal-to-noise ratio is proportional to .sqroot.Q. For low frequency (up to approximately 200 Mhz) the Q can be maintained at a sufficiently high value by the choice of the discrete resistive and reactive components typically employed. As the detection frequency icreases, however, the reactive component values must decrease to maintain the circuit resonance condition, .omega..sup.2 =1/LC where L is the net inductive component and C is the net capacitive component. At high frequencies, L and C can be supplied primarily from the stray inductances and capacitances which couple the detection circuit to resistive circuits which in turn lower the Q.
A further requirement for relatively high Q is recognized from a desire to increase the magnitude of the rotating (RF) field of the excitation. This field is determined by the amplitude of the circulating current in the excitation circuit, which for a matched input can be shown to be proportional to Q.sqroot.P where P is the input power. Thus by using a sufficiently high Q excitation circuit, the input power can be kept low. In this way, sample heating is minimized or avoided, a particularly important advantage where the sample exhibits low dielectric loss. It is also important for samples with high dielectric loss, although in this case the sample itself will lower the cavity Q.
It is therefore apparent that high sensitivity and high Q are critical attributes. It is also essential that access to the sample be convenient.